The present invention relates to a sheet finisher that conducts post-processing for a sheet and to an image forming system equipped with the sheet finisher.
In the past, there has been known an image forming system wherein various types of sheet finishers having functions respectively for punching processing, stapling processing, folding processing and book-binding processing for the sheet subjected to image forming can be mounted on a copying apparatus or a printing apparatus representing an image forming apparatus.
In the image forming system of this kind, there is known a technology to convey two superimposed sheets, for doing post-processing efficiently on sheets which have undergone high speed copying.
Unexamined Japanese Patent Application Publication No. H11-157741 discloses a technology to superimpose two sheets by two branched channels and by stoppers located at downstream sides of the branched channels wherein a sheet positioned to be on the upper side on the stacking section is shifted in a way to precede in the conveyance direction so that sheets are aligned correctly where superimposed sheets are stored temporarily.
The stacking section is arranged to be tilted, and when the superimposed sheets have left the conveyance roller, they are returned along the stacking section in the direction opposite to the conveyance direction by their empty weight and a returning member, to hit the lower stopper to be aligned. However, if the lower sheet is ahead in the conveyance direction, an action of the returning member for the sheet on the upper side does not have an influence on the sheet in the lower side, thus, the sheet in the lower side is stored in the stacking section while keeping the state that the sheet in the lower side is shifted.
To solve this point, in the technology disclosed in Unexamined Japanese Patent Application Publication No. H11-157741, the sheet on the upper side is shifted forcibly so that it may precede in the conveyance direction, and shifting of the sheet is conducted by changing a diameter of a conveyance roller of each branched path, or by changing a speed of rotation of the conveyance roller, or by arranging stoppers at different distances on branched paths, to solve the aforesaid point.
An image forming apparatus has been made to be highly efficient in terms of speeding up of processing speed and of diversification of sheets to be used, and for this trend, a sheet finisher is also demanded to be highly efficient. Under this situation, it has become impossible to obtain sufficient performances, even when the technology in Unexamined Japanese Patent Application Publication No. H11-157741 is used as it is.
Namely, in the technology in Unexamined Japanese Patent Application Publication No. H11-157741, two sheets are shifted by an equal amount of shifting, but an amount of actual shifting varies depending on a type of the sheet, and in a remarkable occasion, a sheet to precede does not precede, and an occasion where adjustment in the stacking section is impossible has come to an actual existence.
In this case, the diversification of sheets means that a range of thickness of sheets that can be used is enlarged and the number of types of sheets that can be used is increased. Further, let it be assumed that the sheet type is a generic name for a sheet thickness (a basis weight, i.e. g/m2) and sheet kinds (coated sheet, color sheet, quality sheet and normal sheet).
For example, in the case of sheets wherein friction force between sheets is large as in normal sheet and sheets wherein friction force is small as in coated sheet, there is dispersion in a shift amount.
Further, if a shift amount is established to be appropriate for a thick sheet, thin sheets slip between them, and a shift amount is varied. On the contrary, when a shift amount is established to be appropriate for a thin sheet, pressing force becomes to be high and a shift amount is reduced.
Dispersion of these amounts of shifting is not large in the case of a low speed copying machine, which has not been a problematic level. However, they are becoming a problem as speeding up and diversification of sheets are carried forward as stated above.